1. Field of the Invention
The invention is related to the field of seismic energy mapping of earth formations. More specifically, the invention is related to methods of determining non-linear elastic properties of earth formations which are geologically interconnected between wellbores by measuring seismic energy transmission properties of the earth formations between the wellbores.
2. Description of the Related Art
Subterranean earth formations sometimes contain zones permeable to fluid flow called reservoirs. Wellbores are drilled into reservoirs for extracting the fluids, which can include commercially useful materials such as petroleum. Wellbore operators are particularly concerned about the geologic structure of reservoirs, as this structure affects the placement of the wellbores used to extract the fluids. Wellbore operators are also concerned with the degree of continuity of hydraulic properties, and the degree of heterogeneity of the reservoir between the wellbores in order to estimate the future volumetric production of fluids from the reservoir.
Various methods are known in the art for inferring the geologic structure of the reservoir and the hydraulic properties of the reservoir from measurements made at the earth's surface, such as reflection seismic surveying. Other methods known in the art include correlation, between wellbores, of measurements made from within each of the wellbores such as measurements made from various types of well logging instruments known in the art.
A method of mapping the geologic continuity between wellbores drilled through earth formations is described in U.S. Pat. No. 5,144,590 issued to Chon. The method disclosed in the Chon '590 patent includes inserting a seismic energy source into one wellbore which penetrates the reservoir, and measuring the seismic energy which travels to receivers placed in another wellbore, which also penetrates the reservoir. The spectral characteristics of the energy which reaches the receivers as it travels from the source can be analyzed to determine if the reservoir is geologically continuous between the wellbores.
The method described in the Chon '590 patent uses seismic energy at a plurality of source frequencies to determine geological continuity of the earth formations between the wellbores. In certain cases the reservoir may be geologically connected between the wellbores, but the hydraulic properties of the reservoir may be discontinuous between the wellbores. Continuity of hydraulic properties is of particular interest to the wellbore operator in order to determine the areal extent of the reservoir, and the ultimate fluid recovery expected from the reservoir.